The present invention relates to radio frequency interference (RFI) shielded plastic articles and enclosures used for electronic equipment. More particularly it relates to RFI shielded non conductive polymeric enclosures which have such electromagnetic shielding on just one side of said enclosure and a process for making the same.
It if often necessary to provide electromagnetic shielding for polymeric and other electrically non-conductive enclosures used in electronic and radio equipment to prevent radiation such as radio-frequency interference from being emitted from the equipment contained therein. Regulations of the Federal Communication Commission require that certain categories of electronic and computer equipment be enclosed by shields capable of substantially eliminating such RFI above about 10 kiloHertz in frequency.
It is known that electrically conducting materials will effectuate electromagnetic radiation shielding in general and RFI shielding in particular. Accordingly it is common practice to provide a metal coating on enclosures for electronic equipment. Among the techniques known to apply such a coating are the use of electrically conductive paints, flame or arc metal spraying, cathode sputtering, vacuum metallization, the application of metal foils and electroless deposition of one or more metal layers. However, to insure tight bonding of the coating to the underlying non-conductive substrate and enclosure it is usually necessary to pretreat the surface by methods involving abrading, etching and similar surface roughening techniques. If not applied with extreme care, such methods can cause discoloration, cracking, distortion, excessive surface stress and in extreme cases, the total destruction of the underlying substrate structure and/or polymeric enclosure. Further, where the enclosure is to be used in an application where only the inside surface is to be coated and where either the exterior surface appearance is to remain unaltered and/or the dimensional tolerances are a matter of some concern, the exterior must be protected in some fashion so as to preserve its color and surface finish. Where electrolessly deposited metallic coatings are involved, this usually involves the application of a resist or other type of strippable coating which will either prevent deposition on the side not to be altered or allow the removal of any deposits which have been laid down. Such procedures, in addition to increasing the overall costs of producing the shielding also tend to degrade to some degree the protected surface so that the finish and/or color may not be entirely suitable for the intended use. In such cases it is necessary to cover up these differences by one or more secondary operations such as mechanically buffing and/or painting the degraded areas. Such approaches, when they work, tend to considerably increase both the time and cost of producing a finished enclosure.